Plant Tissues

Plant Tissues & Organs Cells of a vascular plant are organized into different tissues and organs Three major organs are: roots, stems, and leaves Dermal tissue Vascular tissue Ground tissue Stem Root Leaf

Plant Organs Roots – absorb water and nutrients from soil, anchor plant, hold plant upright Stems – supports plant, transports water and nutrients Leaves – carry out photosynthesis, have adjustable pores to help conserve water, allow O 2 to leave and CO 2 to enter

Roots Two main types – taproots (found in dicots) and fibrous roots (found in monocots –Taproots – long primary root like that of carrots or trees –Fibrous roots – branching root system with no single root growing larger than the rest (many plants, such as grasses)

Root Function Anchors plant Absorbs water and nutrients –Active transport proteins in the root pump minerals and nutrients into the plant (moves from low to high concentration and requires ATP/energy to do so)

Stems Vary greatly in size and shape Three important functions: –Produce leaves, branches, and flowers –Hold leaves up in sunlight –Transport substances between roots and leaves

Growth in Stems Primary growth of stems –For the life of the plant, new cells are produced at the tips of the roots –Plant gets taller Secondary growth of stems –Stem increases in thickness/width –Most evident in woody plants –The vascular cambium is a cylinder of actively dividing cells between the primary xylem and primary phloem. –Secondary growth adds cells on either side of the vascular cambium.

Growth in Stems Key Dermal tissue system Vascular tissue system Ground tissue system Year 1 Early Spring Growth Primary xylem Vascular cambium Cortex Epidermis Primary phloem Growth Late Summer Shed epidermis Secondary xylem (wood) Secondary phloem Cork cambium Cork Bark Year 2 Late Summer Secondary xylem (2 years’ growth)

Formation of Wood in Trees Actually layers of xylem that build up year after year Tree growth is seasonal –In spring, light-colored xylem cells with thin cell walls form –In fall/winter, xylem cells becomes dark and have thicker cell walls –This light and dark pattern is called tree rings. Each ring is composed of a band of light wood and dark wood

Formation of Wood in Trees (continued) –One ring = one year of growth –Counting rings = approximate age of tree –Thick rings = favorable conditions/good growing season –Thin rings = less favorable conditions/bad growing season

Layers of a Tree Trunk Annual rings Heartwood Sapwood Vascular cambium Secondary phloem Cork cambium Cork Bark

Leaves Main organ of photosynthesis Leaves vary in shape, but all collect sunlight on flattened sections called blades Epidermis of leaves covered by a waxy cuticle to prevent water loss

Leaf Functions Photosynthesis – bulk of leaf tissue is called mesophyll, where many chloroplasts are Gas exchange – leaves take in CO 2 and give off O 2 during photosynthesis

Leaf Functions (continued) –Leaves have stomata – pore-like openings on the underside of the leaf that allow gas exchange to occur –Stomata are open just enough to allow photosynthesis to occur, but not so much that they lose excess amounts of water –Guard cells surrounding the stomata control the opening/closing of the stomata and thus regulate the movement of gases into and out of the leaf (also regulates water loss) –In general, stomata open during the day and close at night

Function of Guard Cells Stoma OpenStoma Closed Guard cells Inner cell wall Stoma Guard cells Inner cell wall

Function of Guard Cells Stoma OpenStoma Closed Guard cells Inner cell wall Stoma Guard cells Inner cell wall

The Internal Structure of a Leaf Veins Xylem Phloem Vein Cuticle Epidermis Palisade mesophyll Epidermis Stoma Guard cells Spongy mesophyll

Tissue Types Three tissue types: –D–Dermal –V–Vascular –G–Ground Dermal tissue Vascular tissue Ground tissue Stem Root Leaf

Dermal Tissue Dermal Tissue – outermost layer of cells covered with a waxy cuticle to protect against water loss

Vascular tissue Vascular tissue – transports water and nutrients through plant by xylem and phloem –Xylem – conducts WATER throughout the plant –Phloem – conducts NUTRIENTS throughout the plant

Ground Tissue Ground Tissue – cells that lie between dermal and vascular tissues; site of photosynthesis

The Working Plant: The Transport of Water –To thrive, a plant must be able to transport water and dissolved ions from its roots to the rest of the plant. Xylem is used for this purpose. –Transpiration, the loss of water vapor from the leaves of a plant by evaporation, mostly occurs through the stomata of leaves and pulls liquid through the xylem up the plant against gravity.

The Working Plant: The Transport of Water –Transpiration relies on two special properties of water. 1. Adhesion is the sticking together of molecules of different kinds. 2. Cohesion is the sticking together of molecules of the same kind –Together, adhesion and cohesion create a continuous string of water molecules that stick to each other and to the inside walls of the xylem tubes.

Xylem sap Soil particle Root hair Water Water uptake from soil Root Flow of water Cohesion, by hydrogen bonding Xylem cells Cohesion and adhesion in the xylem Cell wall Water molecule Adhesion Stem Transpiration Leaf Outside air Water molecule Stoma Air space within leaf Mesophyll cells Xylem sap

The Working Plant: The Transport of Water –Transpiration helps to distribute water within a plant but can cause plants to lose large amounts of water. –Plants adjust their transpiration rates to changing environmental conditions.

The Working Plant: The Transport of Sugars –Phloem moves sugar in various directions, from a sugar source, where sugar is produced, to a sugar sink, where sugar is stored or consumed. –Phloem moves sugar from a sugar source to a sugar sink by active transport as well as help from osmosis

The Working Plant: Response to Stimuli –Plants can respond to physical stimuli from the environment, including light, touch, and gravity.

The Working Plant: Response to Stimuli –Tropisms are directed growth responses that cause parts of a plant to grow toward or away from a stimulus.

The Working Plant: Response to Stimuli Phototropism is the directional growth of a plant shoot in response to light. Thigmotropism is a response to touch and occurs when a pea plant tendril coils around a string or wire it touches for support. Gravitropism is the directional growth of a plant organ in response to gravity and occurs when –shoots grow upward and –roots grow downward.

The Working Plant: Response to Stimuli TROPISMS PhototropismThigmotropismGravitropism Seedlings bending toward the light Growth in response to touchSeedlings reacting to gravity

The Working Plant: Response to Stimuli –Light provides energy for photosynthesis, directs growth, and regulates a plant’s life cycle, including –flowering, –seed germination, and –the onset and ending of dormancy. –A photoperiod is the relative lengths of day and night and the environmental stimulus that plants most often use to detect the time of year.